The ball screw is a mechanical element including a plurality of balls arranged between a first rolling groove, formed on the outer circumferential surface of a screw shaft, and a second rolling groove, formed on the inner circumferential surface of a nut externally fit on the screw shaft. The ball screw coverts the rotational power of the screw shaft (or the nut) to a linear power of the nut (or the screw shaft) via the balls. The rolling groove surface of the screw shaft is hardened to improve wear resistance and durability. There are various methods as follows for manufacturing the screw shaft of the ball screw where the hardening is performed by induction hardening.
One of them is a method for manufacturing a screw shaft of a ball screw disclosed in Patent Document 1, listed below, where the rolling groove surface is induction hardened to have a hardness of HRC 55-62 to improve the wear resistance after formation of the rolling groove of the screw shaft is rolled from a blank of middle carbon steel. The screw shaft goes through a buffing process to remove oxidized scale generated by the induction heat treatment. Finally, the screw shaft is straightened to correct deformation of the screw shaft generated during the rolling process and induction hardening process.
Another method of manufacturing a nut screw of a ball screw is disclosed in Patent Document 2, listed below, where oxidized scale is removed from the rolling groove. The rolling groove is finished by performing an electropolishing step on the rolling groove after induction hardening of the rolling groove of the nut screw.
However, in the methods disclosed in Patent Documents 1 and 2, it is an indispensable step to remove the oxidized scale after the induction hardening. This is due to the presence of the oxidized scale on the surface of the rolling groove which causes wear or noise during operation of the ball screw.
Patent Document 3, listed below, discloses a method for removing oxidized scale. However, it is not limited to a method for manufacturing a ball screw. The method is carried out by increasing pressure of cooling water during the induction hardening.
It is required, in the screw shaft of the ball screw, to reduce deflection of the screw shaft after heat treatment as small as possible to prevent the generation of vibration. A plurality of methods have been proposed in the prior art to perform the induction hardening in oil to reduce deformation of the screw shaft after heat treatment and to prevent the generation of oxidized scale. The hardening in oil remarkably reduces the deflection of the screw shaft accompanied with the induction hardening since the screw shaft is uniformly cooled by the oil as a coolant.
In one method of induction hardening in oil, a stationary induction hardening is conducted where the hardening is carried out without causing any relative movement between an induction hardening coil and an article to be hardened. As disclosed in Patent Documents 4 through 6, listed below, there are two methods for stationary induction hardening. In one, both heating and cooling are performed in liquid. In the other, heating is performed in gas and then cooling is performed in a coolant.
Patent Document 4 discloses an induction hardening method where dissolved oxygen in the quenching oil is first removed by performing bubbling of the quenching oil with inert gases, such as nitrogen or argon. The surface of the metallic member is induction hardened in the quenching oil. The metallic member is immediately cooled in the quenching oil to perform the hardening treatment. Thus, the generation of oxidized scale can be prevented by performing cooling while injecting the quenching oil from a cooling jacket using the quenching oil where the dissolved oxygen is removed as much as possible.
Similar to Patent Document 4, where heating and cooling of a metallic member are performed in the quenching oil, in Patent Document 5, the surface of the metallic member, after heating, is cooled by pouring the quenching oil into an oil tank using a cooling nozzle and agitating it.
Patent Document 6 discloses a hardening method where a metallic part is induction hardened in a casing filled with non-oxidizing gas or reducing gas. It is hardened by quickly cooling the metallic part by dipping it into a coolant.
Patent Document 7 discloses a mechanical part, such as a screw shaft of a ball screw with a rolling groove, and its manufacturing method of submerged induction hardening which does not cause black colored oxidized scale.
On the other hand, several devices have been proposed not relating to the induction hardening and adapted to prevent oxidization of a mechanical part to be hardened even in heat treatment using a furnace. Patent Document 8 is one representative example. Patent Document 8 discloses an oil hardening apparatus able to transfer deoxidized quenching oil in a decompressed oil tank equipped, with a vacuum pump, into a quenching oil tank.
Patent Document 9 discloses a continuous annealing apparatus equipped with a formation and degassing apparatus to reduce the concentration of oxygen in the atmosphere within a cooling tank as well as the concentration of dissolved oxygen in the coolant.    Patent Document 1: Japanese Laid-open Patent Publication No. 119518/2003    Patent Document 2: Japanese Laid-open Patent Publication No. 25152/2003    Patent Document 3: Japanese Laid-open Patent Publication No. 129231/2002    Patent Document 4: Japanese Laid-open Patent Publication No. 298213/1990    Patent Document 5: Japanese Laid-open Patent Publication No. 282136/2000    Patent Document 6: Japanese Laid-open Patent Publication No. 1320/1993    Patent Document 7: Japanese Laid-open Patent Publication No. 233288/2006    Patent Document 8: Japanese Laid-open Patent Publication No. 202414/1993    Patent Document 9: Japanese Laid-open Patent Publication No. 11623/1988
In a mechanical part, such as a ball screw, where spherical rolling elements are present, the rolling elements roll in a point-contacting condition (near pure contacting condition). Accordingly, noise would be caused by abrupt change of motion of the rolling elements if foreign matter, such as oxidized scale generated during hardening of the mechanical part, is present on the surface of the rolling groove or bitten therein. Thus, it is an indispensable step for manufacturing a mechanical part, such as ball screw, to remove the oxidized scale.
Oxidized scale is not sufficiently removed from the rolling groove only by the buffing process disclosed in Patent Document 1. An additional problem of the buffing process is that it requires a long processing time and thus increases the manufacturing cost.
Oxidized scale will be perfectly removed from the rolling groove by the electropolishing of the rolling groove after its induction hardening as disclosed in Patent Document 2. However, it has the problems that it requires a long processing time as well as it causes roughness on the surface of rolling groove and softening of the surface layer of the rolling groove due to the chemically reacting heat. In addition, there are problems of treating waste polishing liquid, building facilities and working circumstances which increase costs.
In case of the method for removing oxidized scale disclosed in Patent Document 3, using high pressure cooling water during induction hardening, uniform removal of scale cannot be achieved. Thus, the dotted black scale remains. Accordingly, other scale removing steps, such as a buffing step, is required.
When a deformed ball screw is applied to a transfer apparatus, run-out or vibration will be caused during a high speed rotation of the ball screw. Thus, it is necessary to prevent the generation of oxidized scale after heat treatment as well as to reduce the deformation of the ball screw using a method of induction hardening in oil as disclosed in Patent Documents 4-7.
However since methods of Patent Documents 4 and 5 use quenching oil, with dissolved oxygen removed by an inert gas, it increases the hardening cost due to the use of expensive inert gas.
Also, there is a problem that it is difficult to provide articles of uniform quality of less oxidized scale since it is difficult to control the content amount of oxygen in the quenching oil by the inert gas. Thus, it is difficult to have oxygen-free quenching oil. Accordingly, additional processes, such as shot blasting etc., are required to remove oxidized scale. Thus, irregular surfaces are formed on the articles by shot blasting.
In the method of Patent Document 6, non-oxidizing gas or reducing gas is filled in a casing. Thus, it is necessary to use expensive inert gas. This increases the cost for hardening. In addition, it is also a problem that gas replacement of the casing should be performed. Accordingly, productivity is diminished.
Furthermore, Patent Documents 4-6, stationary induction hardening is conducted where the relative position between a heating coil and an article to be hardened is not changed. Thus, it is a problem that the coil should be fit to the configuration of the article to be hardened. This increases the manufacturing cost.
Patent Document 7 discloses a method of induction hardening in oil a ball screw. It proposes a mechanical part, with a rolling groove, induction hardened in oil without the generating black colored oxidized scale and a method of manufacturing.
Although it is described in Patent Document 7, that the non-oxidizing condition can be achieved, due to the hardening performed in oil containing little oxygen, it is clear as mentioned in Patent Documents 5, 6 and 8 that dissolved oxygen present in the oil will generate oxidized scale. Patent Document 7 does not refer to any method to prevent or remove the dissolved oxygen. Thus, further improvement is required.
Patent Documents 8 and 9 disclose preventing the generation of oxidized scale due to heat in a furnace. The furnace heating requires a long treating time and thus synchronization of the manufacturing line becomes difficult. Accordingly, there is a problem that deformation of the article is increased due to it being held at a high temperature for a long period of time.
Accordingly, there is an occasion where large deflection of an article, caused in the induction hardening step, could never be corrected to necessary dimensions in a later deflection correcting step. Accordingly, it is important to reduce the deflection as small as possible in a later induction hardening step.